The invention relates to a device for the automatic sensing of televiewing distance, and in particular, it relates to a device for the automatic sensing of televiewing distance to sense when a televiewer is located within a given distance in front of the televison. It makes use of the time difference between the time ultrasonic waves emitted from ultrasonic wave transmitting means the television are reflected from an object located in the forward direction of the televison and the time thus the reflected ultrasonic waves are received by ultrasonic wave receiving means.
In general, when a televiewer watches a television set from a short distance in front of it, his eyesight is weakened by the ultraviolet rays produced from the picture on the television.
Therefore, it becomes desirable to detect whether or not a televiewer stays within a given distance in front of the television set, and to advise him that he should move backward a certain distance.
A conventional television, when equipped with ultrasonic transmitting and receiving means on its front face, emits ultrasonic waves from the ultrasonic wave transmitting means where they are reflected from an object located in front of the televison. The reflected waves are then received by the receiving means and a measure of the distance between the object and television is obtained from the intensity of the received waves. Accordingly, such apparatus has an inherent disadvantage in that the density of ultrasonic signals reflected from the object varies greatly depending on the physical properties of the object located in front of the television. Therefore, it is impossible to accurately measure the distance between the television and the object, and thus its reliability is reduced thereby.